In order to successfully implement mission requirements, many spacecraft require active control of the attitude or orientation of the spacecraft body. For instance, an imaging satellite typically utilizes an optical sensor to collect data from regions of interest to the spacecraft operator. In many cases, the imaging sensor is fixed rigidly or semi-rigidly to the spacecraft body. Therefore, in order to align the imaging sensor it is necessary to rotate the entire spacecraft. This method of operation is also common amongst space telescopes such as the Hubble Space Telescope in which the entire spacecraft is maneuvered in order to point the telescope optics and observe an object.
The desired orientation of a spacecraft may be specified in a number of different ways including single-valued functions of time, also known as a step input, or as a set of maneuver command trajectories, in which the desired spacecraft attitude varies as a function of time. Conventionally, maneuver command trajectories have been generated directly on-board the spacecraft by implementing a so-called command generator as part of the spacecraft flight control software. One advantage of utilizing an on-board command generator is that attitude command inputs can be easily computed at a rate that is consistent with the sampling rate of the attitude control system. This action prevents large discontinuities in the command trajectory from one sample time to the next and helps to attenuate or prevent the excitation of flexible modes inherent of the spacecraft structure or payload.
In order to generate a trajectory of spacecraft attitude commands, the command generator can read a time-tagged desired attitude setpoint from a storage buffer, also located on the spacecraft. The time-tagged desired attitude setpoint is uplinked to the spacecraft from a ground station by the spacecraft operator. After reading the desired attitude setpoint from the storage buffer, the onboard maneuver generator is utilized to construct a trajectory between the current spacecraft position and the desired final position. The trajectory can be a straight line, or any curve.